After you have chosen your motherboard, then you should choose your RAM. The reason for choosing the board before the RAM is because certain boards work better with certain memory kits. It has to do with how strong of a relationship each motherboard vendor has with each memory vendor, as they will swap hardware to ensure maximum compatibility between products and can fix bugs if they arise. Each motherboard vendor puts out a Qualified Vendor List (QVL) or certified memory list either on their webpages as a downloadable document or inside the motherboard manual.

The kits listed in these lists are fully compatible with the motherboard and the Threadripper CPUs. I would use the list for your motherboard to find a compatible kit before purchasing your memory. If you don't care for memory speed or are going for very high density (128GB+), I would stick with 2400-3000MHz for memory speed. If you are going for pure speed and are all good with 32GB or 64GB, then I would look for a kit that's 3200MHz to 3600MHz.

Threadripper does much better with single rank kits rather than dual rank kits when it comes to density and overclocking. In dual rank kits, memory chips are split into two sets, where one can refresh while the other is being used, and they do provide a bump in performance (a noticeable amount in benchmarks). Single rank kits will have a 1RxN (N is the number is memory chips, typically 8) in their specs while dual rank will have 2RxN, and most of the time single rank kits have memory only on one side of the stick while dual rank typically has chips on both.

As you can see the official AMD supported memory speeds (guaranteed) are 2667MHz with four sticks of single rank, 2133MHz with eight sticks of dual rank, 2400MHz with four sticks of dual rank, and 1866MHz with eight sticks of dual rank. These numbers might seem low, as we can easily get 32GB (4x8GB) to 3200MHz and even 3600MHz with Threadripper, but AMD can't guarantee that (and shouldn't as it's overclocking).

You should keep in mind that there are many different types of DDR4, including ECC DIMMs which are error correcting and rely on an extra chip per group of DIMMs that acts as a parity to correct any errors (flipped bits caused by things like natural phenomenon). Threadripper supports unregistered ECC DIMMs (center), dual rank DDR4 DIMMs (right), and single rank DDR4 DIMMs (left). Now, AMD has a lot of resources for tuning your RAM; you can find blog posts by AMD here and here that offer a lot of insight into single vs. dual rank, speeds, timings, AMD memory settings, and overclocking.

Samsung B-die is the preferred memory chip type for overclocking, and right now they are the easiest to overclock, and resourceful Reddit users compiled a very nice list of memory DIMMs on the market and what type of chips they use; you can find it here. I should also mention that you should check the motherboard vendor's specifications to see if they have fully implemented ECC support, as not all of them have (some will support it but not implement it). I don't care for ECC support since these days we have other mechanisms to maintain memory integrity, but I also don't work on mission-critical data where ECC is a must. I recommend a 3200MHz dual rank kit (they aren't very common) if you can find one or a 3600MHz single rank kit, I recommend Samsung B-die for both if you want to overclock.

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